Systems and methods of treating water used for hydraulic fracturing

ABSTRACT

A system and method of treating a fluid to be used for hydraulic fracturing adds an effective amount of chlorine dioxide to the fluid to act as a biocide that kills harmful bacteria. A system for adding chlorine dioxide to the fluid can continuously add chlorine dioxide to an incoming flow of the fluid to produce a continuous flow of treated fluid.

This application is a divisional application of U.S. application Ser.No. 15/150,228, filed May 9, 2016, is a continuation of U.S. applicationSer. No. 14/106,052, filed Dec. 13, 2013, now issued as U.S. Pat. No.9,358,517, which is a continuation of U.S. application Ser. No.14/020,495, filed Sep. 6, 2013, now issued as U.S. Pat. No. 8,962,534,which itself claims priority to U.S. Provisional Application No.61/697,932, filed Sep. 7, 2012, the entire contents of which are herebyincorporated by reference.

BACKGROUND

Hydraulic fracturing refers to a process in which a wellbore is drilledinto a rock formation, and a fracturing fluid is then pumped into thewellbore under great pressure to induce an initial set of fractures inthe rock formation. The fracturing fluid is then forced into the initialfractures, extending the fractures further. An operator may thenintroduce a proppant into the fracturing fluid that is being pumped intothe fractures, such as sand, ceramic particles to other particulates.The proppant is designed to prevent the fractures from closing when thefracturing fluid pressure is reduced or when pumping stops.

Once the fracturing process is complete, petroleum, natural gas andother fluids which are under considerable pressure in the rock formationescape through the fractures and into the wellbore. The petroleum andnatural gas is ultimately extracted through the fractures and thewellbore and is captured and stored. During the initial stages after thefracturing is completed, the petroleum and natural gas may push much ofthe fracturing fluid back up the well bore. However, it is also commonfor a significant percentage of the fracturing fluid to disperse intothe surrounding rock through the fractures that were created.

The fracturing fluid which is pumped into the wellbore typicallyincludes water. However, various chemicals and materials are added tothe water for various purposes. The chemicals that are added can dependupon the type of well and the underground rock formations that are beingfractured. In addition to the proppants mentioned above, frictionreducing additives can be used to increase the flow rate and to reducethe pressure needed to pump the fracturing fluid into the well. Oxygenscavengers and other stabilizers and corrosion inhibitors can be addedto prevent corrosion of the pipes. Acid, such as hydrochloric acid, canbe added to help dissolve minerals and induce fractures. Surfactants andcrosslinking agents help to maintain fluid viscosity, particularly astemperature increases. Gels can be added to help suspend greater amountsof proppants in the fracturing fluid. A scale inhibitor may be added toprevent scale deposits in the pipes. A pH adjusting agent may be addedto maintain the effectiveness of the other agents, particularly thecrosslinkers. Finally, various biocides may be added to eliminatebacteria which can produce corrosive byproducts that attack the pipes.The biocides that are added to fracturing fluid can includeglutaraldehyde, quaternary amines, active liquid brominated propionamidesolutions and other materials. It is important that any biocide added tothe fracturing fluid not disrupt or inhibit the functions performed bythe other additives.

Because the fracturing fluid can escape into the surrounding rockformations, and ultimately propagate into the water table, there isgreat concern about the chemical additives that are present in thefracturing fluid. Thus, operators are seeking to use chemicals which areknown to be safe to humans, animals and the environment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is flow diagram illustrating a method of treating a fracturingfluid with a biocide;

FIG. 2 is a diagram of a system for treating a fracturing fluid with abiocide; and

FIG. 3 is a diagram of an alternate system for treating a fracturingfluid with a biocide.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The inventor has discovered that chlorine dioxide can be used as aneffective biocide in fracturing fluids. Chlorine dioxide in appropriateconcentrations can be as effective, or more effective, in eliminatingharmful bacteria in fracturing fluid than known biocides used for thispurpose, such as glutaraldehyde, quaternary amines, and active liquidbrominated propionamide solutions. In addition, chlorine dioxide is anEPA approved material for use as a biocide because it is not harmful tohumans, animals or the environment when used in the low dosages andconcentrations required for effective use as a biocide.

Chlorine dioxide can be formed in many different ways. Some of thereactions which produce chlorine dioxide result in the formation of freechlorine, which is undesirable. However, the inventor has determinedthat an effective reaction that produces chlorine dioxide that can beused as a biocide in fracturing fluid, and which does not result in theproduction of free chlorine is the reaction formula:

5 NaClO₂+4 HCl→5NaCl+4ClO₂+2H₂O.

A flowchart illustrating a method embodying the invention for treating afluid that is to be used for hydraulic fracturing is presented inFIG. 1. As shown in FIG. 1, the method begins in step S102, where asupply of chlorine dioxide is either obtained or generated. If thechlorine dioxide is to be generated, it is preferably generatedaccording to the reaction formula illustrated above. In alternateembodiments, a supply of chlorine dioxide could be generated in otherways. In still other alternate embodiments, a supply of chlorine dioxidecould simply be obtained from a source.

In step S104, the obtained or generated chlorine dioxide is added to aflow of fluid which is to be used for hydraulic fracturing. The chlorinedioxide is added in an amount that results in the concentration of thechlorine dioxide in the fracturing fluid being high enough to act as aneffective biocide to kill harmful bacteria that may be present in thefracturing fluid. However, the concentration should also be low enoughthat the residual chlorine dioxide present in the fracturing fluid willnot be harmful to humans, animals or the environment. The concentrationmust also remain low enough to prevent the chlorine dioxide frominterfering with the functions performed by the other additives andchemicals that are present in the fracturing fluid. Desirable residualconcentrations are generally within the range of 0.1 to 0.8 parts permillion.

The inventor has developed a treatment system which continuously mixeschlorine dioxide into an incoming flow of fluid which is to be used forhydraulic fracturing. Thus, the treatment system developed by theinventor can perform a method as illustrated in FIG. 1. Once thetreatment system has added chlorine dioxide to the incoming flow, theflow can be immediately used for hydraulic fracturing, or the treatedfluid can be collected in a storage tank before the treated fluid isactually pumped into a wellbore during a hydraulic fracturing process.

A first embodiment of such a treatment system is illustrated in FIG. 2.As shown therein, a fluid supply pipe 110 receives a flow of fluid thatis to be used for hydraulic fracturing. The fluid flows along the supplypipe 110 to an outlet end 118 which delivers the fluid to a fracturingfluid storage tank 130. As the fluid flows along the supply pipe 110,chlorine dioxide is added to the fluid by a chlorine dioxide treatingunit 120.

As shown in FIG. 2, a header 112 installed in the fluid pipe 110includes an inlet pipe 113 that is attached to an inlet 121 of thechlorine dioxide treating unit 120. An outlet pipe 115 which is attachedto an outlet 123 of the chlorine dioxide treating unit delivers treatedfluid back into the supply pipe 110 via the header 112.

In operation, a relatively small percentage of the fluid flowing throughthe supply pipe 110 is routed through the header 112, the inlet pipe 113and the inlet 121 into the chlorine dioxide treating unit 120. Chlorinedioxide is added to the received fluid, and the treated fluid is sendback into the supply pipe 110 via the outlet 123, outlet pipe 115 andthe header 112. The treated fluid mixes with the fluid in the supplypipe 110 as it travels along the supply pipe 110 to the outlet 118 andinto the fracturing fluid storage tank 130.

The fluid in the supply pipe 110 is typically under pressure. The flowof fluid through the inlet pipe 113, the chlorine dioxide treating unit120 and the outlet pipe 115 will generate flow losses which result inthe fluid in the outlet pipe being at a lower pressure than the fluid inthe supply pipe 110. In order to force the treated fluid from the outletpipe 115 back into the fluid supply pipe 110, it is necessary to raisethe pressure of the fluid in the outlet pipe 115. A booster pump 125located on the inlet pipe 113 can be used for this purpose. However, thebooster pump 125 could be located at alternate positions, such as withinthe chlorine dioxide treating unit 120, or on the outlet pipe 115.

In some embodiments, the chlorine dioxide treating unit 120 may receivea supply of chlorine dioxide that is added to the fluid. In alternateembodiments, the chlorine dioxide treating unit 120 may include a firstsupply reservoir 122, a second supply reservoir 124, and a third supplyreservoir 126. The first supply reservoir 122 contains a supply ofsodium chlorite, and the second supply reservoir 124 contains a supplyof hydrochloric acid. The third supply reservoir 126 contains a supplyof fresh water. The chlorine dioxide treating unit 120 utilizes thesodium chlorite, hydrochloric acid and fresh water to generate chlorinedioxide which is then added to the fluid.

In some embodiments, the chlorine dioxide treating unit 120 may includea flow meter. Alternatively, a flow meter could be installed on theheader 112, the inlet pipe 113, or the outlet pipe 115. In such anembodiment, the flow meter would indicate the flow rate of the fluidpassing through the chlorine dioxide treating unit 120, and thisinformation could be used to determine the appropriate amount ofchlorine dioxide to add to the fluid.

In alternate embodiments, a test header 116 having a sample pipe 117 isconnected to the fluid supply pipe 110 downstream from the header 112.The sample pipe 117 conveys a sample of the treated fluid back to thechlorine dioxide treating unit 120, which checks the concentration ofchlorine dioxide in the fluid collected through the sample pipe 117. Thechlorine dioxide treating unit can then selectively vary the amount ofchlorine dioxide being added to the fluid to achieve the properconcentration of chlorine dioxide in the fluid. By sampling the fluidfrom a location well downstream of where the treated fluid is deliveredback into the supply pipe 110, it is possible to ensure that the treatedfluid delivered into the supply pipe 110 via the outlet pipe 115 hasfully mixed with the fluid in the supply pipe 110 such that a finalresulting chlorine dioxide concentration has been achieved.

As also illustrated in FIG. 2, in some embodiments control valves 142and 144 are installed on the inlet pipe 113 and outlet pipe 115,respectively. A controller 140 is coupled to the control valves 142, 144and to the chlorine dioxide treating unit 120. In this embodiment, thecontroller 140 selectively varies the flow rate of the fluid through thechlorine dioxide treating unit 120 to control the amount of chlorinedioxide that is added to the fracturing fluid in the supply pipe 110.The controller 140 may also, or alternatively, exert control over theamount of chlorine dioxide that the chlorine dioxide treating unit 120is adding to the fluid. Thus, the controller 140 could be used ensurethat the appropriate amount of chlorine dioxide is being added to thefluid.

In some instances, a control valve 146 may also be installed in thesample line 117 that is connected to the test header 116. In thoseembodiments, the controller 140 may also be coupled to the control valve146 to control the flow of fluid through the sample line 117.

The fluid being delivered into the supply pipe 110 can be acquired frommany different sources in different fracturing operations. Often thefluid is acquired from a local lake or pond, from a well, or the fluidis delivered via a tanker truck. In some instances, it may be possibleto acquire water from a local water supply system operated by amunicipality or a city. Because the fluid can be acquired from manydifferent sources, there can be different amounts of bacteria or otherbiological contaminants which are to be partially or fully eliminated bythe chlorine dioxide treatment.

The chlorine dioxide that is added to the fluid is basically consumed inchemical reactions in order to eliminate the bacteria or otherbiological contaminants that are present in the fluid. Thus, some or allof the chlorine dioxide added to the fluid in the supply pipe 110 viathe chlorine dioxide treating unit 120 will be consumed before the fluidis ultimately used in a fracturing operation.

It is desirable for the residual amount of chlorine dioxide that ispresent in the fluid ultimately used in a fracturing operation to be inthe range of 0.1 to 0.8 ppm. However, one cannot simply add an amount ofchlorine dioxide to the fluid in the supply pipe 110 in a predeterminedvolume amount to achieve this concentration because some of the chlorinedioxide that is added to the fluid will be consumed in eliminatingbacteria and other biological contaminants that were present in thefluid being supplied into the fluid supply pipe 110. And because one cannever be certain how much bacteria or other biological contaminants arepresent in the supply water, one can never know in advance how muchchlorine dioxide will be consumed.

Because of these factors, it is desirable to test the fluid in the fluidfracturing storage tank 130, or perhaps even farther downstream in thesupply chain to determine the actual residual chlorine dioxideconcentration in the fracturing fluid once some of the chlorine dioxidehas been consumed in eliminating bacteria and other biologicalcontaminants. If the residual chlorine dioxide concentration is too low,the chlorine dioxide treating unit 120 can be adjusted to insert greateramounts of chlorine dioxide into the fluid, or vice versa. Periodictesting may be required to continually adjust the amount of chlorinedioxide that is being added to the fluid in the fluid supply pipe 110 toensure that the residual chlorine dioxide concentration in the fluidbeing used in fracturing operations is within the desirable range.

In some embodiments, the sample pipe 117 may be located sufficiently fardownstream that the bacteria and other biological contaminantsoriginally present in the supply water will have been largely eliminatedby the chlorine dioxide before a sample of the fluid is drawn throughthe sample pipe 117. Thus, measuring the concentration of the chlorinedioxide in the fluid drawn from the sample pipe 117 may provide theinformation needed to continually adjust the amount of chlorine dioxidebeing added by the chlorine dioxide treating unit 120 to achieve adesirable residual chlorine dioxide concentration. This may make itpossible for the controller 140 to provide automated control of theamount of chlorine dioxide that is added.

In other embodiments, manual samples of the fluid may be drawn fromother locations that are farther downstream, such as from a locationjust before the fluid is used in a fracturing operation. In thatinstance, it may be necessary for an operator to manually adjust theamount of chlorine dioxide that is being added by the chlorine dioxidetreating unit 120.

FIG. 3 illustrates an alternate embodiment of the system which issimilar to the one discussed above in connection with FIG. 2.Accordingly, only the differences will be discussed.

In the embodiment illustrated in FIG. 3, the booster pump 125 is locatedon the outlet line 115 connected to the chlorine dioxide treating unit120, instead of the inlet line 113. Also, instead of a single header112, first and second headers 112, 114 are used to draw fluid from thesupply pipe 110, and return treated fluid to the supply pipe 110,respectively. Also, the sample pipe 117 connected to the test header 116is coupled to an analyzer unit 150. The analyzer unit 150 determines aconcentration of chlorine dioxide in the fluid obtained through thesample pipe 117. The analyzer unit 150 then sends a signal to thechlorine dioxide treating unit 120 via a control line 154, the signalbeing representative of the determined chlorine dioxide concentration.The chlorine dioxide treating unit 120 uses the signal to selectivelyadjust the amount of chlorine dioxide being added to the fluid toachieve a desired residual chlorine dioxide concentration in thefracturing fluid.

In an alternate embodiment, the analyzer unit 150 may be coupled to acontroller 140 via a signal line 152, so that a signal representative ofthe determined chlorine dioxide concentration is received by thecontroller 140. The controller 140 may then instruct the chlorinedioxide treating unit 120 to selectively adjust the amount of chlorinedioxide being added to the fluid. Alternatively, or in addition, thecontroller 140 could utilize the signal from the analyzer unit 150 toselectively vary a flow rate through the chlorine dioxide treating unit120, via one or both of the control valves 142, 144 to adjust the amountof chlorine dioxide being added to the fluid.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A method of treating a fluid used for hydraulicfracturing, comprising: obtaining chlorine dioxide; adding an amount ofthe obtained chlorine dioxide to a continuously supplied fluid which isto be used for hydraulic fracturing such that the chlorine dioxide actsas a biocide, wherein a prescribed amount of the chlorine dioxide isadded per unit volume of the continuously supplied fluid; periodicallytesting the continuously supplied fluid after the chlorine dioxide hasbeen added and after at least some of the chlorine dioxide has beenconsumed in reactions with bacteria and/or biological contaminants thatwere originally present in the fluid to determine a concentration of thechlorine dioxide in the supplied fluid; and selectively adjusting theprescribed amount of chlorine dioxide that is added per unit volume ofthe continuously supplied fluid based on the result of the testing stepsuch that a residual concentration of chlorine dioxide remaining in thefluid after the chlorine dioxide has had an opportunity to react withbacteria and/or biological contaminants is in the range of 0.1 to 25parts per million.
 2. The method of claim 1, wherein the adjusting stepis performed such that a residual concentration of chlorine dioxideremaining in the fluid after the chlorine dioxide has had an opportunityto react with bacteria and/or biological contaminants is in the range of0.1 to 0.8 parts per million.
 3. The method of claim 1, wherein the stepof obtaining chlorine dioxide comprises reacting sodium chlorite withhydrochloric acid to produce chlorine dioxide.
 4. The method of claim 1,wherein the reaction that produces the chlorine dioxide proceedsaccording to the following chemical equation:5 NaClO₂+4 HCl→5NaCl+4ClO₂+2H₂O.
 5. The method of claim 1, wherein theadding step comprises: diverting a portion of the continuously suppliedfluid from a main supply pipe to a treatment unit; adding the prescribedamount of the obtained chlorine dioxide to a unit volume of the divertedfluid to create a flow of treated fluid; and delivering the flow oftreated fluid back into the main supply pipe.
 6. The method of claim 5,wherein the periodic testing is performed on fluid drawn from the mainsupply pipe at a location that is downstream from where the treatedfluid is delivered back into the main supply pipe.
 7. A method oftreating a fluid used for hydraulic fracturing, comprising: obtainingchlorine dioxide; diverting a predetermined flow rate of a continuouslysupplied fluid that is to be used for hydraulic fracturing from a mainsupply pipe to a treatment unit; adding a substantially constant flowrate of the obtained chlorine dioxide to the diverted flow of fluid tocreate a flow of treated fluid; delivering the flow of treated fluidback into the main supply pipe; periodically testing fluid drawn fromthe main supply pipe at a location that is downstream from where thetreated fluid is delivered back into the main supply pipe, and after atleast some of the chlorine dioxide has been consumed in reactions withbacteria and/or biological contaminants that were originally present inthe fluid, to determine a concentration of the chlorine dioxide in thetested fluid; and selectively adjusting the flow rate of the fluid thatis diverted from the main supply pipe into the treatment unit based onthe result of the testing step such that a residual concentration ofchlorine dioxide remaining in the fluid after the chlorine dioxide hashad an opportunity to react with bacteria and/or biological contaminantsis in the range of 0.1 to 25 parts per million.
 8. The method of claim7, wherein the adjusting step is performed such that a residualconcentration of chlorine dioxide remaining in the fluid after thechlorine dioxide has had an opportunity to react with bacteria and/orbiological contaminants is in the range of 0.1 to 0.8 parts per million.9. The method of claim 7, wherein the step of obtaining chlorine dioxidecomprises reacting sodium chlorite with hydrochloric acid to producechlorine dioxide.
 10. The method of claim 9, wherein the reaction thatproduces the chlorine dioxide proceeds according to the followingchemical equation:5 NaClO₂+4 HCl→5NaCl+4ClO₂+2H₂O.